Odo says the Bajorans had to smuggle the aphasic device into the station while it was under construction, before he became security chief. But the station was built 18 years ago, and Odo only became Security Chief 9 years ago. So they had ages to plant the device before he came along.

Plotline

An old Bajoran booby trap infects the residents of Deep Space Nine with a virus that causes aphasia - and, ultimately, death.